In the early fetal period of mice, hematopoiesis is carried out in the yolk sac and fetal liver. Hematopoiesis in the yolk sac is referred to as fetal hematopoiesis during which primarily nucleated fetal erythrocytes are produced. On the other hand, hematopoiesis in the fetal liver is referred to as adult hematopoiesis during which all lines of blood cells are produced with the exception of nucleated fetal erythrocytes.
Although the activity that causes the production of all lines of blood cells, that is the long-term repopulating hematopoietic stem cell (LTR-HSC) activity of hematopoietic stem cells, is not detected in fetal hematopoiesis, it is detected in adult hematopoiesis. It is now thought that the cells having this LTR-HSC activity are actually produced not initially in the liver, but in the aorta-gonad-mesonephros (AGM) region at day 10-11 of embryogenesis. During this period, these cells are thought to also proliferate in this AGM region, after which they migrate to the fetal liver (Medvinsky et al., Cell 86:897-906). Thus, a gene that is important for the generation of hematopoietic stem cells may be expressed in this AGM region.